Beyond the Blue - Water Colour 101

Series Hub: Reading the Sea the Old Fashioned Way

Subject: Water colour, turbidity, depth indicators, tidal fronts, and what colour change means for the navigator

Reading Water Colour at Sea — Why the Sea is Blue, Green, or Brown, and What Each Tells the Navigator

Fill a glass from the sea. The water in it is perfectly clear and colourless. Look out across the same sea, and what you see is grey, blue, green, brown, or some imprecise muddle of all four. The sea does not have a colour the way a painted wall has a colour. What you are looking at is the combined result of depth, particles, the seabed, and the light. Every one of those variables is carrying information you can use.

This note is about learning to read that information. Most of what follows draws on Tristan Gooley’s chapter on water colour in How to Read Water, which works through the physics more carefully than I have seen elsewhere.

Why deep water is blue, and shallow water is not

Pure water is not colourless in the way air is colourless. It absorbs light, but it does not absorb all wavelengths equally. Red, orange, and yellow are absorbed strongly. Blue and blue-green wavelengths travel furthest before being absorbed. The result is that the more water light has to travel through, the bluer it appears when it re-emerges. In a glass, the path length is too short to produce any visible effect. In a swimming pool, you begin to see a faint blue tint on the white bottom. In the deep ocean, with sunlight travelling through tens of metres, the red end of the spectrum has been completely stripped out and what you are left with is the deep indigo-blue that the phrase blue-water sailing actually refers to.

Gooley notes that the wavelength that penetrates furthest through clean water is 480 nanometres, a blue-green, and that deep, clear, oligotrophic water, low in nutrients and low in algae, approximates this colour most closely. The Mediterranean is a useful example. Unusually nutrient-poor compared to the North Atlantic or North Sea, with relatively little phytoplankton, the water is extraordinarily clear, and that characteristic deep blue is close to the pure physics of it.

Shallow water behaves differently because the seabed contributes. Gooley describes it as a colour palette. Deep blue from above, mixed with whatever the bottom is. White sand adds white, giving turquoise or pale aquamarine. Sandy seabed adds yellow, giving blue-green. Mud adds brown, giving the greenish-brown that characterises many inshore estuaries. The shallower the water, the more the bottom dominates and the more the depth-blue fades from the equation.

This creates a depth gauge that costs nothing to use. The colour gradient from pale turquoise to deeper blue to deep indigo over a white-sand bottom is a direct read of the gradient in depth, which matters considerably if you are approaching an unfamiliar anchorage in the Canaries, the Mediterranean, or anywhere with good visibility and a sandy seabed. In northern European waters the visibility is rarely good enough for this to be precise, but the broad principle still applies. The darker the water, the deeper you are likely to be.

What particles do to the colour

Most water you sail in is not pure. It carries particles. Sediment, algae, clay, organic matter, chalk, mineral runoff. Every one of these shifts the colour away from the pure-physics blue, and each shift tells you something.

Gooley describes the Forel-Ule scale, a classification of water colour from indigo blue (number one, open-ocean deep water low in everything) through various greens and greenish-browns to cola brown (number twenty-one, heavy estuary water with high humic-acid concentrations from decaying plant matter). The scale is a useful mental framework even if you never use the actual test tubes.

Indigo through greenish-blue is open offshore water. Low nutrients, low organic load, the colour governed mainly by depth and the pure-water physics of light absorption. Green is the typical signature of coastal water with elevated nutrients, phytoplankton, and some suspended matter. The English Channel in summer runs distinctly green rather than blue. Greenish-brown to brownish-green is the colour of nutrient-rich water with high phytoplankton and increased sediment. Near-shore areas, tidal flats, anywhere with significant river influence. Brown to cola-brown is the colour of river and estuary water itself. High humic acids from organic material, high silt load. The Thames, the Humber, the Rhine plume spreading into the North Sea. All of them sit at this end of the scale.

In practice, sailing in northern European waters, the greenish-blue to green transition marks the move from offshore to coastal water fairly consistently. The North Sea runs notably greener than the deep water west of Scotland or in the Western Approaches, partly because it is shallower and therefore has more suspended sediment in circulation, partly because nutrient runoff from the surrounding heavily farmed land keeps phytoplankton levels elevated year-round. The Baltic is greener still, often with a distinctly blue-green cast inshore, and after significant algal-bloom events in summer can go visibly cloudy-green over wide areas. I have sailed into a light greenish haze in the Baltic in August that I initially took for mist until I realised the visibility was perfectly fine. The water colour was tinting the lower light.

River plumes and tidal fronts as navigational features

One of the most practically useful things water colour does is mark boundaries. Places where two different bodies of water meet and have not yet fully mixed.

River plumes extend seaward from estuary mouths as a distinct wedge of darker, more turbid water that remains visually separate from the surrounding sea for considerable distances. The Thames plume at certain states of tide and wind is visible from a good way offshore as a browner, murkier body of water against the surrounding Channel green. If you know what the water should look like offshore here, a change tells you something is different, and usually that something is a boundary.

Tidal fronts behave similarly. Where two different water masses meet, different temperatures, different salinities, different ages, they often form a visible line at the surface. The line may be a colour change, a texture change, a line of foam and floating debris (material collects at convergence zones), or a combination. Fishing vessels know these lines well because fish concentrate along them. From a navigational standpoint they mark real boundaries between hydrographic regimes and are sometimes marked on pilot charts as areas of interest.

Off the west coast of Scotland, the boundary between coastal water and the cleaner Atlantic water to the west is occasionally visible as a distinct colour change. Greener inshore, bluer offshore. I have not been far enough west from the Hebrides to see it definitively, but there are areas around the Outer Hebrides where the water does noticeably change character on a sunny day. In good visibility in the Western Approaches, the cleaner water west of the Scilly Isles has a markedly different quality from the Channel water to the east.

The angle of observation

There is a piece of geometry that Gooley explains clearly and that changes how you interpret what you are seeing. When you look at water at a low, glancing angle, the way you look at the sea on the horizon, you cannot see into it at all. What you see is entirely reflection. The sky, and what is beyond the horizon behind you, reflected back at a shallow angle. This is why the sea on the horizon looks blue on a sunny day and grey on an overcast one. It is not the water’s colour you are seeing. It is the sky’s.

When you look nearly straight down into water, as you do over the side of the boat in a calm anchorage, the reflection effect is reduced and you can see into the water itself, reading the depth and the bottom type.

Between these extremes there is a transition zone, somewhere between twenty and thirty degrees from vertical, where the view shifts from looking into the water to looking at the reflection. This matters if you are trying to read the bottom for depth or bottom type. You need to be close to the water and looking nearly straight down, not admiring the view from the cockpit.

It also explains a common confusion. The dark patches moving across the sea surface that many sailors initially take for shoals or sudden depth changes. Gooley notes this as one of the most frequently misread effects in water. They are nearly always cloud shadows. The tell is that they move with the clouds. Track a patch, look up for the parent cloud, draw the line. If it is a cloud shadow, the cloud will be on that line between you and the sun.

Algal blooms, foam, and abnormal colour

A significant departure from the expected colour of the water in any given area is worth noting. The most common cause in European coastal waters is an algal bloom. A rapid multiplication of phytoplankton driven by elevated nutrients, warmth, and light. In summer in the North Sea and Baltic, blooms can turn extensive areas of water a vivid green or, in the case of some species, a red-brown. The red tide is a particular bloom type that can be toxic to shellfish and sometimes to humans, and is an occasional hazard in inshore waters around Scotland and Ireland.

Gooley points out that foam is always white regardless of the water’s colour, because foam is essentially air bubbles which scatter all wavelengths of light equally. The navigational observation. Foam that persists for an unusual length of time indicates surfactants. Detergents and industrial chemicals that stabilise the bubbles. Long-lasting foam near a harbour entrance, river mouth, or industrial shoreline is a mild pollution indicator.

Post-storm water is often temporarily discoloured by turbulence lifting the seabed. Gooley suggests studying the water before and after a storm in familiar coastal areas. The return to normal colour over a day gives a sense of the water’s flushing rate, which tells you something about tidal energy and mixing in that area.

Depth, bottom type, and the navigator’s tradition

There is a North Sea fisherman quoted in the 1880s, in a passage Gooley includes in How to Read Water, who put this with the straightforwardness of someone who could not imagine why anyone would find it remarkable.

"There’s nothin’ in the world can be easier, when you’ve learned your lesson, than to pick your way about in the North Sea just with nothing else to guide yer than the depth o’ water an’ the natur’ o’ the bottom."

He was talking about navigating by lead line and bottom sample. The technique covered fully in The Lead Line — Depth Sounding. But the same information is partly available visually, before you ever cast a lead. The colour of the water and its turbidity give you the broad-brush version of the chart. Blue is deep and clear. Green is moderate coastal. Brown is shallow and turbid. Any sudden change from one to another is a boundary worth paying attention to.

Pacific Island navigators used water colour for chart-quality navigation around coral atolls. David Lewis records in We, the Navigators that an experienced local skipper’s eye for colour changes in shallow reef water still beats any electronic chart of the same area, where GPS and chart accuracy is often poor. The principle extends to any area where local knowledge of what the water normally looks like makes departures from that norm readable. This is less critical in European waters with good charts and reliable GPS. It becomes directly relevant when those tools fail or when you are approaching somewhere unfamiliar in poor conditions.

The colour of the water as you approach a coastline, combined with depth from a lead line, is how sailors navigated the southern North Sea for centuries. As that fisherman suggested, it was not considered difficult once you had learned what to look for in your particular waters. The chart annotations that seem archaic, S for sand, M for mud, Sh for shells, Oz for ooze (which is a wonderful word to have kept in use), exist precisely because these bottom types produce different colour signatures in the water above them, and different holding qualities for an anchor. The foreshore version of the same reading is in What the Foreshore Tells You.

Light on the surface — glitter paths as a sea-state indicator

A brief note on one of Gooley’s more striking observations, which deserves inclusion here because it is directly useful on watch. The glitter path, the bright shimmering road that the sun or moon throws across the water toward you, is not just beautiful. Its width is a measure of the roughness of the water.

A narrow glitter path means calm, flat water. As waves steepen, their faces present reflected light at a wider range of angles, and the glitter path broadens. If you are watching a glitter path during a night passage or an early morning watch, any section conspicuously wider than the rest is rougher water. Indicating perhaps a current line, a shallow patch, or a local wind effect worth noting. Gooley describes watching the glitter path from Falmouth Harbour broaden in the middle where a tidal current was running, making a rough patch visible at night purely through this effect. Polarised sunglasses suppress the glitter path significantly. Worth remembering if you are specifically trying to use it. The night-time use of the same effect is in What the Sea Does at Night.

Putting it together at sea

The practical reading habit is not complicated. As you sail into new water, note the colour. Compare it to what you expect from the chart. Depth, proximity to land, any river influence. If the colour is what you expect, that is confirming information. If it is not, if you are getting green water where the chart suggests forty metres of blue offshore water, or if you see a sudden darkening or lightening that cannot be a cloud, something is different from what the chart is telling you, and it is worth investigating.

In the Thames Estuary approaches, the distinctive brownish-green water of the river influence is a navigational feature you can read from seaward. In the Channel in summer, the green-blue transition from coastal to offshore water is real and sometimes visible. In the Western Approaches west of the Scillies, the quality of the water changes character. Cleaner, deeper blue, often visibly different from the Channel water east of the islands.

None of this replaces a depth sounder, a chart, and a tidal atlas for close-quarters navigation. It adds a layer of observational intelligence available continuously and for free, requires no power, and never loses its signal. On a passage in poor electronic conditions, a lightning strike, a flooded chart table, a corrupted chart card, the colour and texture of the water is one of the first things that still works. The wave-side context for the same observation is in What Waves Know, and the current side in What Moving Water Tells You.

What I’m still working out

Most of the colour reading I have field-tested is in the green-to-brown range. The Channel, North Sea, Thames Estuary, Baltic, the Western Approaches as far as the Scillies. The blue end of the scale, the deep oligotrophic clarity of the open Atlantic and the Mediterranean, I have less direct experience of, and the Caribbean and Pacific reef-water reading I have read about but not done. The framework transfers cleanly in principle. The local calibrations, what counts as normal in a particular sea area, do not. They have to be built in the water you actually sail in.

If you want to start that calibration somewhere sheltered enough to watch the colour change with the tide and the light, the Hithe Finder is a community register of slipways, hards, and beaches suitable for small boats. An hour at a launch site on a falling tide, watching the same water at high water and low, in sun and in cloud, builds the eye for what is normal. Which is the only way to recognise what is not.

Gooley, T. (2016). How to Read Water: Clues and Patterns from Puddles to the Sea. Sceptre. Includes the chapter on water colour with the Forel-Ule scale, the physics of light absorption, the angle-of-view geometry, glitter paths, the cloud-shadow tell, and the 1880s North Sea fisherman quotation.

Lewis, D. (1994). We, the Navigators: The Ancient Art of Landfinding in the Pacific, 2nd edition. University of Hawai’i Press. Pacific Island traditions of reef-water and lagoon-colour reading as a primary navigational tool.

Lane, C.D. (1942, reprinted 2011). The Boatman’s Manual: A Complete Manual of Boat Handling. Practical small-craft references for inshore observation and approach.

Related notes in the series. Depth and bottom-type sounding in The Lead Line — Depth Sounding. The foreshore version of bottom-type reading in What the Foreshore Tells You. The wave context in What Waves Know. The current context in What Moving Water Tells You. The night version of glitter-path reading in What the Sea Does at Night. The wind-and-fetch context in The Beaufort Scale and What It Actually Looks Like. The full Reading the Sea series index is at Reading the Sea the Old Fashioned Way.

At VAKA I design and build boats that don’t destroy the environment. Find the plans as they are finalised at VAKA Plans and the full field notes here.

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